Motion of the Earth and Ethe)\ suggested hg the late Professor FitzGerald. 381 



As the condenser plates lay horizontally, the efFect should have been a maxi- 

 mum at twelve o'clock. The experiments were therefore carried out between the 



hours of eleven and one each day. In ^ ^^ ^ 



order to have the maximum turning ^^^^p^% 



effect the arm was jilaced north and ^ -^-JIZ^ 



south. 



Some preliminary experiments were 

 made with a view of testing if a blow, 

 whose kinetic energy was onl}' 1 erg, 

 could be competent to produce observa- 

 ble effects. This was done by causing 

 a small object to strike against the 

 arrangement. 



These proving highly encouraging, 

 apparatus was constructed with the 

 object of enabling the condenser to be condene 



charged and discharged continuously 



by means of clock-work at the intervals 



corresjoonding to tlie free period of Fig. I. 



swing of the apparatus. In this way any effect produced would cumulate and 



be made easier of observation. 



The complete period of swing was about 60 seconds, so that the charging and 

 discharging followed each other at half these intervals or about 30 seconds. 

 Coincidence between the rate of the clock which drove the commutator and that 

 of the apparatus was most readily effected by increasing or diminishing the 

 moment of inertia of the suspended apparatus, by means of small weights laid 

 on it or removed as required. This coincidence in period was made very perfect. 



The source of the current for charging the condenser was a continuous- 

 current dynamo separately excited, capable of supplying a current of ^ ampfere 

 at 1200 volts. Relays were employed for charging and discharging the 

 condenser, as it was found undesirable, being likely to affect the rate of the clock, 

 to allow such heavy sparking as necessarily took place to occur on the clock- 

 driven commutator. 



The leads for the charging current were provided by the suspending wire and 

 by a wire which dipped centrally downwards into a mercury cup. A curved 

 mirror was affixed to provide a spot of light on a mm. scale at the distance 

 of 1860 mm. 



The method of experimenting was to charge and discharge at proper intervals 

 by means of the synchronous commutator, and to observe if a swing was set up. 

 Another plan sometimes used was to cause the apparatus to have a small swing to 



